1. Field of the Invention
The present invention relates to the tools and methods for earth boring and deep well completion. In particular, the invention relates to tools, materials and operational methods for placing an annulus of cement around a pipe or tube along a defined length of well bore.
2. Description of the Related Art
A well annulus is that generally annular space within a wellbore that may be between the raw borehole wall and the outside of a casing pipe suspended within the borehole. The term may also be applied to the annular space between the raw borehole wall and the outside surface of a fluid production tube. The well annulus may also be that annular space between the casing inside surface and the outer surfaces of a pipe or tube that is suspended within the casing.
Packers are well completion tools that are used to segregate axially adjacent sections of the well annulus to prevent the transfer of fluids, liquid or gas, from flowing along the length of an annulus from one section to another or migrating from one earth strata to another. More generally, the packer is a structural barrier across an annulus section that usually extends along a short length of the annulus.
Characteristically, inflatable packers comprise an elastomer or rubber sleeve element around the outer perimeter of a tubular mandrel. Opposite ends of the elastomer sleeve are secured to the mandrel. The tubular mandrel wall provides structural strength to physically link elements of a tubular work string above and below the packer. Additionally, the open bore along the mandrel center provides working fluid (hydraulic oil, etc.) flow continuity from surface located pumps to other tools below the packer.
The opposing ends of a packer sleeve may be overlaid by collar elements. One or both collars may include valve devices to admit pressurized fluid from the mandrel flow bore into the interface between the elastomer sleeve and the outer surface elements of the mandrel. Sufficient pressure within the interface expands the elastomer radially from the mandrel surface out to a tight, pressure seal against the internal walls of the annulus to prevent fluid flow in either direction along the annulus past the packer.
A wellbore zone to be produced through the flow bore of a production tube or casing liner is often isolated by an annular collar that is cast in cement around the production tube or casing liner. The cement collar is also cast in intimate contact with the surrounding borehole wall or inside surface of the casing bore. This collar seals the wellbore annulus around the casing liner and also secures the casing liner within the wellbore.
A prior art procedure for placement of the uncured collar cement within the well annulus includes placement of form packers in the well annulus above and below the collar segment. For downhole placement, the packers are tool segments of the well casing liner that are secured within the casing liner pipe string at positions of axial separation corresponding to the desired length of the cement collar. Between the packers, the casing liner (or production tube) may also include a pair of selectively opened and closed cement valve elements for providing respective cement flow paths between the flow bore of the casing liner and the surrounding annulus. By means of a cementing tool, a cement flow path between one of the cement valves and the tubular flow bore of the cement tool is isolated. Cement is pumped from the surface, along the cementing tool flow bore, through transverse flow ports in the cement tool, and into the annulus around the casing liner. The other cement valve in the casing liner string receives the material in the collar annulus that is displaced by the uncured cement. This displaced material is received into an inner annulus between the cementing tool and the interior of the casing liner.
A raw borehole profile often is irregular. Although the exact dimension of the outside casing liner dimensions are known, the unknown volume within the borehole prevents a precise determination of the annulus volume between the collar packers. Consequently, a considerable excess of cement is pumped into the collar annulus simply to assure that the collar annulus is filled. Any excess cement flows through the second cement valve into the inner annulus between the casing liner interior and the cementing tool exterior. Removal of the cementing tool swabs the casing liner bore of the excess cement.
A major difficulty of the foregoing prior art process is the unknown. Notwithstanding delivery of volumetrically excessive cement, there is no certainty that the collar annulus is completely filled. It is therefor, an objective of the present invention to provide equipment and procedures to positively conclude a volumetric filling of a collar annulus.
This and other objects of the invention as will become apparent from the following detailed description are obtained by a procedure that includes a shrouding screen over the cement return (ingress) valve. The cement egress valve is positioned along the casing liner or production string, as the case may be, between the pair of collar delineating packers but closely proximate of one. The screen shrouded return valve is also positioned between the packers but closely proximate of the other packer.
In cooperation with a liner casing or production tube having a shrouding screen over the cement ingress valve, the cement injected into the collar annulus is blended with a particulate or compatible thixotropic material that is matched to the mesh or slot opening of the shrouding screen.
Fluids within the collar annulus that are volumetrically displaced by a pressure driven influx of cement have a traditional drain route through the cement ingress valve and covering screen. However, when the particulate blended cement reaches the screen element over the cement ingress valve, the particulates will not pass through the screen openings. In due time, most of the screen mesh or slot opening will be bridged over by the cement borne particulates. A well working crew at the surface will recognize the condition by an increase in the cement pump discharge pressure as a consequence.